DESCRIPTION: The events that control the precise connections formed by motor neurons are critical for establishing the neural control of movement. Developmental studies of motor neurons have begun to define the molecular events that trigger their differentiation, control their survival, and mediate the formation of synapses. By contrast, little is known about the molecular mechanisms that guide motor axons to their appropriate targets. Cellular studies of motor neurons provide evidence that distinct subtypes of motor cells are generated during development. Each subtype of motor neuron appears to express unique factors that mediate selective axonal projections. The overall aim of this project is to examine the molecular basis of motor axon guidance, and define genes that control this process. Our preliminary studies have shown that a family of five LIM homeobox genes are expressed in combinatorial arrays in distinct subclasses of motor neurons. LIM genes are thought to encode transcription factors, and the expression patterns of these genes suggest they could regulate the transcription of genes that mediate axonal responses to guidance cues. Based on these studies, the proposal has two major goals: (1) to characterize LIM homeodomain protein expression in motor neurons, (2) to examine the role of combinatorial LIM gene codes for differentiation and axonal projections of motor neurons. The characterization of LIM homeodomain protein expression in defined motor neurons will be addressed by using immunocytochemistry, retrograde neuronal labeling, and spinal cord grafts. These experiments provide background information for our functional studies of these genes using mouse genetics. The differentiation and axonal projection of motor neuron subtypes in Isl-2 null embryos will be examined using marker gene expression and neuronal labeling. The role of combinatorial LIM gene expression for axonal targeting will be addressed in two genetic models. First, Lim-3 and Gsh-4 function will be eliminated from MMCm neurons by targeted mutation. Second, Lim-3 will be ectopically expressed in non-MMCm motor neurons. Following these genetic alterations of LIM gene coding, the applicants will examine the differentiation and axonal projections of motor neuron subclasses. In the long term these studies will provide insight into the molecular mechanisms that contribute to the formation of neural circuits that underlie locomotion. This information may provide novel methods for intervention to restore motor function lost due to motor neuron injury or disease.